Process for the production of a curved laminated glass pane

ABSTRACT

Laminated glazing for automotive use in which a thermoplastic functional film is thermoformed and then cooled by forced draught, prior to the introduction between the glass sheets of the laminated glazing.

This application is based on and claims priority to European ApplicationNo. 03425381.5, filed on Jun. 12, 2003 and International Application No.PCT/EP2004/051106, filed on Jun. 14, 2004 designating the U.S., theentire contents of both of which are hereby incorporated by reference.

This invention relates to laminated glass panes which incorporate atransparent functional film embedded between the laminating layers, andto methods for the manufacture of such panes. The invention findsparticular application in the manufacture of laminated automotiveglazings, i.e. windshields, sidelights, backlights and rooflights or inother fields in which curved glazings may be employed, e.g. in thebuilding industry.

Laminated glass panes comprise two or more sheets of glass, bondedtogether with an interlayer therebetween, comprising one or more layersof a bonding resin, which is typically polyvinylbutyral (PVB). The glassis usually an inorganic glass but rigid transparent organic materials,e.g. polycarbonate, may also be used. The resin used to bond the glasssheets may provide safety properties or establish a bond with one ormore additional layers that are incorporated between the glass sheets toprovide the end product with improved properties of strength,shatter-resistance and the like.

Commonly, a layer of PVB or ethylene vinyl acetate having a thickness inthe range of 0.2 mm to 1.0 mm, typically 0.38 mm or 0.76 mm, isincorporated in the end product, e.g. to provide an automotivewindshield having acceptable safety properties.

Frequently, laminated glass panes incorporate also a functional film asadditional layer, e.g. a thermoplastic layer having peculiar physicalproperties, in combination with said interlayer. Most commonly, thisfilm is embedded between two or more layers of a bonding resin, such asPVB, since the PVB is known to adhere well to the glass than a possiblefunctional layer. Interlayer having a more complex structure, i.e. twodifferent layers of functional film or alike therebetween, separated byan inner layer of bonding resin and placed between two outer layers ofbonding resin, may be also used.

These functional films are used to impart additional properties to thepane, such as solar control properties, and in particular a filteringfunction for dazzling wavelengths, to decrease solar heat transmission,or to provide heating means or an increased safety.

An example of a material which is used to provide a functional film ispolyethylene terephthalate (PET), possibly coated with furthersub-layers for achieving said additional properties.

A typical laminated pane comprising a functional film formed from PET isdescribed e.g. in U.S. Pat. No. 4,799,745.

One difficulty, which may arise from the incorporation of a functionalfilm into a laminated pane, is the production of a laminate, which istransparent and free from any optical defects. The conventionallaminating process using, for example, a layer of PVB as the bondingresin, comprises the step of heating the laminate inside an autoclave toa softening temperature for the PVB resin, allowing the resin to flow,thereby forming a transparent film having no significant opticaldefects. Generally, the functional thermoplastic films have a highermelting point than the PVB resin, thus they are not softened to the samedegree at the PVB softening temperature. The result may be theproduction of a composite laminate having a creased or wrinkledappearance.

Such laminates are unacceptable for most uses, especially for automotiveglazings.

This problem is particularly felt in relation to curved laminates suchas automotive windshields, especially those having a high degree ofcross curvature, where the risk to obtain a laminate having a wrinkledappearance is even greater. Moreover, the modern automotive designrequires glazing surfaces with ever-increasing curvatures.

European patent application EP 877,664 A describes a process for theproduction of a laminated glass pane comprising a PET film as part ofthe interlayer, in which the PET is stretched prior to the laminationprocess in order to impart specific thermal shrinkage properties to thePET. The PET-containing interlayer is placed between two glass sheetsand any excess is trimmed off prior to the final lamination step. Suchprocedures have been found to alleviate but not completely remove theproblem of wrinkling, especially when the laminate is a curved laminatewith a high degree of cross curvature.

U.S. Pat. No. 5,208,080 discloses a laminated composite glass panehaving a functional film, having smaller dimensions than each glasspane. The gap surrounding the functional film is filled with a strip ofan auxiliary film, preferably a strip of PVB. The use of such auxiliaryfilm strip has been found disadvantageous in that it promote theformation of air bubbles and other distortions at the boundary of thefunctional film and the auxiliary strip.

In the International patent application Publ. No. WO 00/26023, coatedlaminated glass windshields are described, having an interlayercomprising a thermoplastic functional film wherein the edge of thefunctional film lies within a peripheral obscuration band.

An improved process, described in the International Application Publ.No. WO 01/51279, allows to drastically reduce wrinkles and creaseswithout using any auxiliary film, reducing the size of the functionalfilm by a peripheral trimming, whereby the possible wrinkles areeliminated and the interlayer outer edge lies within the edge of theglass panes.

In this case, a thin obscuration band, producing an opaque or partiallyopaque contour, may mask the uncoated reduced peripheral region.

Anyhow, such a contour can reduce, though very slightly, the transparentsurface of the glazing, and such a process could be less than effectivewhen the curvature of the end product exceeds certain values.

U.S. Pat. No. 5,025,895 describes a process in which a compositeinterlayer is pre-produced by gluing layers of bonding resin tofunctional layers. Subsequently, this composite interlayer isthermoformed and used to produce laminated glazings. However, thesecontrivances are complex and cannot always avoid the formation ofwrinkles with reduced curvature radiuses.

U.S. Pat. No. 5,264,058 describes instead a process for manufacturinglaminate glazings in which a plastics material layer is preformed athigh temperature and, without cooling, employed to couple glass sheets,using the latter as heat sink to extract heat from the plastics materiallayer. Subsequently the glazing panel is treated in autoclave to obtainthe final product However, also these contrivances are complex andcannot always avoid the formation of wrinkles with reduced curvatureradiuses.

European patent application EP 0,326,015 A describes a process formanufacturing an interlayer in which the functional film is thermoformedand subsequently coupled to layers of bonding resin to adhere to glasssheets, however, in this case as well, the formation of wrinkles withreduced curvature radiuses cannot always be avoided.

At last, the International Application Publ. No. WO 91/19586 discloses aprocess wherein a composite interlayer is laminated with all the endlayers, thermoformed on a mould and then allowed to cool. Again, the endproduct is not completely satisfactory.

Thus, from one aspect this invention provides a process for theproduction of a curved laminated glass pane comprising a first glasssheet and a second corresponding glass sheet, together with aninterlayer comprising at least one bioriented thermoplastic functionallayer, i.e. pre-stretched according to two directions substantiallyperpendicular therebetween in order to prevent wrinkling, and acorresponding at least one layer of a bonding resin having a reducedlevel of optical defects due to creasing of the interlayer, such processcomprising the steps of positioning said interlayer between the twoglass sheets and applying pressure and heat to form a laminated paneshowing an end shape with one or more curvatures, which process ischaracterised in that said at least one bioriented thermoplasticfunctional layer is thermoformed on a mould together with said at leastone layer of a bonding resin, in a configuration substantiallycorresponding to the end shape of said curved laminated glass pane, andthen cooled by forced draught prior to being positioned between saidglass sheets to form said interlayer.

In a preferred embodiment, the interlayer may comprise one or morefunctional layers cold-overlapped prior to thermoforming and thenthermoformed in a single cycle, by heating to a softening temperatureadequate for all the functional layers, depositing on a mould andapplying vacuum between mould and interlayer, followed by a finalcooling by forced draught.

According to another preferred embodiment, the functional layercomprises a peripheral pre-cut portion apt to be removed from theinterlayer. In this embodiment, one bonding resin layer is adhered toone face of said functional layer, the latter being subsequentlycold-stamped in a configuration substantially corresponding to the endshape of said curved laminated glass pane, placed a mould and heated andthen cooled by forced draught. This thermoforming step allows thethermoplastic functional layer to be permanently shaped after thesubsequent cooling by forced draught.

The resulting interlayer is positioned on a glass glazing adhering thebonding resin to the glass surface, and said precut portion is peeledoff, whereby the outer edge of the functional layer remains to a certaindistance from the edge of the glazings. Then, a further bonding resinlayer is applied on the exposed functional layer surface beforepositioning a second glass glazing on it and applying pressure and heatto form a laminated glazing showing an end shape with one or morecurvatures.

The invention finds particular application in the production of curvedlaminated automotive glazings, especially windshields, which have a highdegree of cross curvature, say even over 20.0 mm. It is also applicableto glazings which have a relatively small radius; e.g. at least onecurve having a radius of less than 10.000.0 mm and possibly less than10.0 mm at least one point on their surface.

This process allows to obtain glazings free from any optical defectseven at their edges, making peripheral obscuration bands superfluous.

Two embodiments of the invention are hereinafter described, by way ofexample and not for limitative purposes with reference to theaccompanying drawings, wherein:

FIG. 1 is a schematic exploded view of a windshield constructed with theprocess according to one embodiment of the present invention;

FIG. 2 schematically illustrates the composition of a functional film tobe used in the process according to one embodiment of the presentinvention;

FIG. 3 schematically illustrates a step of the process according to oneembodiment of the present invention; and

FIG. 4 illustrates another step of the process according to the presentinvention; and

FIG. 5 is a schematic exploded view of a windshield constructedaccording to a second embodiment of the present invention.

By reference to the FIGS. 1 to 4, the present process for the productionof a curved laminated glass pane has the object of providing a pane 1comprising a pair of shaped glass sheets 2 and showing a shape havingone or more curvatures.

Between the glass sheets, as mentioned in the foregoing, an interlayer 3that carries out a two-fold task is introduced. In fact, the interlayer3 according to this embodiment comprises three layers: in particular,two layers 4 of a bonding resin positioned into contact with said glasssheets 2 in order to provide the adhesive bond strength required to keepthem joined and one layer of functional layer 5.

Said bonding resin is preferably polyvinylbutyral (PVB).

For functional layer it is meant a layer carrying out a predeterminedfunction, e.g. a filter with particular optical properties to abate theradiant solar energy crossing the glass pane 1, a coloured layer tocolour the glass pane, a partially reflecting layer, a conductive layerprearranged for the thermal heating of the glass.

A film in polyethylene terephthalate (PET) with very reduced thicknessesprovides adequate support for the functional layer. Other adequatesupports are formed from other thermoplastic resins.

In the case of materials having thermal shrinkage properties, as in thecase of PET, the film for the functional layer is advantageouslybioriented, i.e. it has been stretched according to two axessubstantially normal therebetween.

In this embodiment (FIG. 2), the selected functional layer comprises afilm in PET, indicated by 6, with various performance sub-layersdeposited on one surface of the PET. In particular, there are depositedthree filtering sub-layers 7 in metal oxide and two reflectingsub-layers 8, in silver, alternated thereamong.

However, it is understood that the number of performance sub-layers inbonding resin, of functional layers and of glass sheets may be varied inorder to meet contingent needs.

The process according to this embodiment comprises thermoforming theinterlayer 3, i.e. the layers 4 in bonding resin and the functionallayer 5.

Said layers are suitably pre-laminated and wound on a roll 11 of awinding machine. They are unwound and inserted between a top moulding 12and a bottom moulding 13, positioned above a curved mould 14 (FIG. 3).

The films are fastened to the mouldings 12, 13 by a snap fasteningsystem, the edge subjected to pinching will have a thickness of 15-20mm.

At this stage, the layers 4, 5 blocked between the mouldings 12, 13 areremoved from the mould and subjected to heating, say by a radiant panelheating system 15 with infrared lamps.

The heating temperature is set at 130° C.+/−30° C.

A temperature detection system is advantageously operated, using aninfrared pyrometer 16 with a sensor of wavelength typical of theabovedescribed plastic materials, which detects the temperature rise ofthe film overlapping. As soon as the temperature reaches the presetvalue an electric command signal activates a system that interrupts theheating, i.e. retracts the heating panels from the films.

According to a preferred version, during the heating, a hot airjet isinjected from the bottom so as effect a film pretensioning prior topositioning them on the mould.

Then, after a blowing time of from 1 to 3 seconds, the mould raisesbottom to top or lowers top to bottom forcing the layer, held stationaryby the two mouldings 12, 13, to assume its shape. All of thesethermoforming steps are carried out under vacuum, thereby furtherfostering the formation of the layer resulting from the overlapping ofthe abovedescribed films (FIG. 4).

After some seconds of pressing by vacuum application, a cold airjet isactivated that cools said layer to a temperature of about 30-40° C.,freezing the shape of the functional layer 5 and, consequently, of thebinding resin layers 4.

After this cooling, operated by forced draught, the two mouldings 12,13, which held the interlayer 3 stationary along all these steps,reopen, and the operator can recover the shaped interlayer 3 which, inorder to better retain the impressed shape characteristics, may bestowed on a support having the same shape of the mould used for thethermoforming.

The thermoformed overlapping of the functional layer 5 and of the layersin bonding resin 4 can be used to manufacture curved glass panesaccording to usual techniques, yet with the option of employing veryreduced curvature radiuses.

Therefore, the process according to this embodiment further comprisesthe step of positioning said interlayer 3 between the two glass glazings2 in a suitable autoclave, wherein appropriate pressure and heat areapplied, to form a laminated pane characterised by an end shape with oneor more curvatures.

This step realizes the binding between the sheets of glass, by virtue ofthe adhesion of the bonding resin layers 4 to the respective glazingsurface.

The peripheral margin of the interlayer 3 shows no wrinkles or creases,which therefore may not need to be masked by a dotted area impressed onthe inner surface of the external glass glazings (i.e., in theautomotive of building field, the glazings intended to be placed at theexternal side) or on the inner surface of the internal glass glazings(i.e., in the automotive of building field, the glazings intended to beplaced at the internal side).

A second embodiment of the present process for the manufacture oflaminated glass panes which incorporate a transparent functional filmembedded between the laminating layers is hereinafter described.

According this embodiment, the interlayer 3′ is composed by onefunctional layer 5 as above specified, and by one bonding resin layer,adhered to one face of the functional layer.

Along its whole edge, the functional layer comprises a pre-cutperipheral portion 5′ (FIG. 5), apt to be removed in a subsequent stepof the present process.

Before the thermoforming and the cooling steps of the precedingembodiment, the interlayer 3′ is cold-stamped, i.e. is pre-formed atroom temperature. This cold-stamping step involves the biorientedthermoplastic functional layer 5 and the corresponding layer of abonding resin, which are shaped in a configuration substantiallycorresponding to the end shape of the curved laminated glass pane to bemanufactured.

For the thermoforming step, vacuum is applied to the interlayer 3′, tomake it adhere to a suitable mould, having a shape substantiallycorresponding to the end shape of the curved laminated glass pane to bemanufactured. The interlayer 3′ is positioned with the functional layer5 perfectly adherent to the mould surface.

By appropriate heating means, the interlayer 3′ is heated to a softeningtemperature of the thermoplastic material of the functional layer, i.e.a temperature of 130° C.+/−30° C.

The heating phase can be carried out by a radiant panel heating systemwith infrared lamps, with a temperature detection system using aninfrared pyrometer 16 with a sensor of wavelength typical of the abovedescribed thermoplastic materials, which detects the temperature rise ofthe film overlapping. As soon as the temperature reaches the presetvalue an electric command signal activates a system that interrupts theheating, i.e. retracts the heating panels from the films.

After some seconds, a cold airjet is activated that cools said layers toa temperature of about 30-40° C., freezing the shape of the functional.Thus, the bioriented thermoplastic functional layer 5 is permanentlyshaped by the cooling by forced draught.

After the above described thermoforming and cooling steps, theinterlayer 3′ is positioned over one glass glazing, which is intended tobe an internal glazing, i.e. intended to be placed at the internal sideof the final glass pane. The bonding resin layer 4 is applied to theglass surface.

At this stage, said pre-cut portion 5′ is peeled off, whereby the outeredge of the functional layer 5 remains to a certain distance from theedge of the receiving glazing.

Then, a further bonding resin layer 4 is applied at least on the exposedfunctional layer 5 surface, and a second glass glazing is positioned onit, the shape thereof perfectly matching with the shape of the firstglazing and of the interlayer 3′ covered by said additional bondingresin layer 4.

In this connection, the composite glass pane according to the presentinvention reaches its final composition (FIG. 5). This semi-finishedproduct is now placed into appropriate means for applying pressure andheat (e.g. an autoclave) to form a laminated glazing showing an endshape with one or more curvatures.

According to the second embodiment, the functional layer 5 shows alinear edge near to the glazing edge, which is almost intelligible andwhich can easily hidden by a peripheral dotted area impressed on theinner face of the inner glazing. This configuration is easier to beobtained than that with a peripheral dotted area impressed on the innerface of the outer glazing.

Moreover, since the functional layer is kept to a distance from theexternal edge of the final glass pane, it is effectively protected bythe corrosion due to the infiltration of atmospherical agents or thelike between the glazings at the edge thereof.

To the abovedescribed process a person skilled in the art, in order tosatisfy further and contingent needs, may effect several furthermodifications and variants, all however encompassed by the protectivescope of the present invention, as defined by the annexed claims.

1. A process for the production of a curved laminated glass pane comprising a first glass glazing and a second corresponding glass glazing, together with an interlayer comprising at least one bioriented thermoplastic functional layer and at least one layer of a bonding resin, such process comprising the steps of: thermoforming on a mould said at least one bioriented thermoplastic functional layer, together with at least one layer of a bonding resin adhered to said at least one bioriented thermoplastic functional layer, in a configuration substantially corresponding to the end shape of said curved laminated glass pane; positioning said interlayer between the two glass glazings and applying pressure and heat to form a laminated glazing showing an end shape with one or more curvatures; and wherein, before the thermoforming step, said at least one bioriented thermoplastic functional layer and at least one layer of a bonding resin are heated and, during such heating, a hot air jet is injected from the bottom so as to effect a pretensioning of said at least one functional layer and then, after the thermoforming step and before said positioning said interlayer between the two glass glazings, said at least one functional layer and said at least one layer of a bonding resin are cooled by forced draught, whereby the shape of said at least one functional film is frozen.
 2. A process according to claim 1, wherein said interlayer comprises two bonding resin layers, intended to be positioned into contact with distinct glass glazings, each adhered to the opposite side of one functional layer.
 3. A process according to claim 1, wherein said interlayer comprises one functional layer, and a corresponding one bonding resin layer, adhered to one face of the functional layer, the functional layer comprising, along its whole edge a pre-cut peripheral portion, apt to be removed in a subsequent step.
 4. A process according to claim 3, wherein, before the thermoforming and the cooling steps, said interlayer is cold-stamped in a configuration substantially corresponding to the end shape of the curved laminated glass pane to be manufactured.
 5. A process according to claim 4, wherein, in the thermoforming step, vacuum is applied to the interlayer to make it adhere to the mould with the functional layer adherent to the mould surface.
 6. A process according to claim 3, wherein, in the positioning step, the shaped interlayer is positioned over one glass glazing, with the bonding resin layer is applied to the glass surface.
 7. A process according to claim 6, wherein said one glass glazing is intended to be at the internal side of the final glass pane.
 8. A process according to claim 7, wherein said pre-cut portion is peeled off when the interlayer has been positioned over said one glass glazing, whereby the outer edge of the functional layer remains to a certain distance from the edge of said one glazing.
 9. A process according to claim 8, wherein, in the positioning step, a further bonding resin layer is applied at least on the exposed functional layer surface, and a second glass glazing is positioned on it, the shape thereof perfectly matching with the shape of said one glazing and of the interlayer covered by said additional bonding resin layer.
 10. A process according to claim 1, wherein, before the thermoforming and the cooling steps, said interlayer is cold-stamped in a configuration substantially corresponding to the end shape of the curved laminated glass pane to be manufactured.
 11. A process according to claim 1 wherein said bonding resin is polyvinylbutyral (PVB).
 12. A process according to claim 1 wherein the glass pane is a curved glass pane having a cross curvature of at least 5.0 mm.
 13. A process according to any claim 1 wherein the glass pane is a curved glass pane wherein at least a part of the curved surface has a radius of less than 10000 mm.
 14. A process according to claim 1 wherein said functional layer comprises a film in polyethylene terephthalate with one or more filtering and/or reflecting sub-layers adhered thereto.
 15. A process according to claim 1 wherein in the thermoforming the heating temperature is set at 130° C.+/−30° C.
 16. A process according to claim 1 wherein in the thermoforming there is employed a temperature detection system with use of an infrared pyrometer with a wavelength sensor which detects the temperature rise of said at least one bioriented thermoplastic functional film and said suitable layers of a bonding resin and, as soon as the temperatures reaches the preset value, activates a system that interrupts the heating.
 17. A process according to claim 3, wherein, in the thermoforming step, vacuum is applied to the interlayer to make it adhere to the mould with the functional layer adherent to the mould surface.
 18. A process according to claim 4, wherein, in the positioning step, the shaped interlayer is positioned over one glass glazing, with the bonding resin layer is applied to the glass surface.
 19. A process according to claim 5, wherein, in the positioning step, the shaped interlayer is positioned over one glass glazing, with the bonding resin layer is applied to the glass surface.
 20. A process according to claim 6, wherein said pre-cut portion is peeled off when the interlayer has been positioned over said one glass glazing, whereby the outer edge of the functional layer remains to a certain distance from the edge of said one glazing.
 21. A process for the production of a curved laminated glass pane comprising a first glass glazing and a second corresponding glass glazing, together with an interlayer comprising at least one bioriented thermoplastic functional layer and at least one layer of a bonding resin adhered to the at least one biorented thermoplastic functional layer, the process comprising: contacting the at least one functional layer with a mould so as to thermoform, on the mould, the at least one functional layer together with the at least one layer of a bonding resin, in a configuration substantially corresponding to an end shape of the curved laminated glass pane; positioning the interlayer between the two glass glazings and applying pressure and heat to form a laminated glazing showing the end shape; and prior to contacting the at least one functional layer with the mould, heating the at least one functional layer while injecting hot air so as to effect a pretensioning of the at least one functional layer. 